My Proton has less than 40K on it so I can't really speak to the longevity of the engine.
"Also, what kind of complications are there with getting payment/title? is it all trust based?"
It doesn't have to be trust based. You should create your own sales agreement to be signed by both parties. It doesn't have to be in a high level of legal jargon. Just make sure all of the issues are mentioned.
Once mine was shipped from MO to VA & the title came by registered mail, it still took several weeks to get the title transferred to my name (there was still a lien against the title - it had been paid off but the lien was never removed). The sales agreement had a statement in it that the seller was responsible to ensure the title was transferrable so he had to get a letter of lien satisfaction from the lien holder. The sales agreement also had a statement that the seller was responsible for ensuring that the vehicle could be picked up by the transport company (that helped too).